Need Optical Expert

[loc46south]

Hi - Still having problems with my GSO 12" R/C Truss - I have obtained and read Souters Book and am now more confused than when I started.

The telescope has been collimated with a Tak Collimation Scope, the scope was rotated to make sure it was correctly centred and that the secondary was correctly aligned. Focuser fitted was a Moonlite CFL 2.5 motorised with all screw fittings.

The pre focus donut clearly shows an off centre reflection in the middle. That reflection is also deformed. I suspected that the spider was off centre but I borrowed a large set of engineers calipers and it is not, well not according to them.

Can anyone help me understand what is going on here as something is out of alignment :help:- The dot in the centre is it caused by the primary mirror, the secondary mirror or a combination of both. :screwy:

Thanks
Geof Wingham

[Paul Haese]

Are you sure you could see all the mirror clips when using the Tak scope? You need to make sure you can see all of them and evenly on the edge.

Next, this image is not useful for determining collimation. One or two Freznel rings are all that are needed.

My guess is that the primary is tilted one way and the secondary tilted the other.

[Star Catcher]

Hi Geof

Tend to agree with Paul. Although the distortion of the energy spot in the centre is a bit odd, almost suggestive of pinched optics, however the donut looks pretty symmetrical. Did you test with a good laser to see if return beam hits the centre spot? The following thread on Cloudynights might shed further light: http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6549517/Main/6544222

Ted

[loc46south]

It is pinched optics - don't have a good laser but the Takahashi Collimation scope is the one recommended by RCOS for collimating their scopres. It seems reliable. Thanks for the C/N Thread sheds a bit more light on the subject.

[loc46south]

Hi Paul - I didn't know if it was movement of the primary mirror or not - Image was taken after I had cut out the silicon holding the primary into the cell. Image attached taken pre silicon removal, shows that collimation with Tak scope reasonably reliable - Scope was re collimated after silicon removal and it apprears the weight of the camera and filter wheel has shifted the primary mirror. Not surprising considering the primary uses a rubber ring as a washer and the way the focuser and camera are attached.

Thanks
Geof Wingham